Grey matter in key areas of the brain seems to be linked to intelligence. Size may matter after all, when it comes to IQ. A brain imaging study suggests that human intellect is based on the volume of grey matter in certain brain regions, challenging alternative views about the basis of intelligence.

Researchers have been trying to pinpoint the biological roots of intelligence for decades. More than 25 years ago, a weak correlation was found between IQ and overall brain size. Others have suggested that level of intelligence is due to the size of the frontal lobe. Now, however, a common view is that more subtle characteristics are likely to be involved, such as the speed at which nerve impulses travel in the brain, or the number of neuronal connections present.

This study challenges that idea, suggesting that the volume of certain brain regions may have an effect after all.Richard Haier from the University of California, Irvine, and colleagues used magnetic resonance imaging to measure the amount of grey matter in the brains of 47 adults, who also took standard IQ tests.

Scoring the brainThe researchers divided the brain into sections and imaged the amount of grey matter in each one. Grey matter is a diffuse network of brain regions thought to be involved in information processing.

It is rich in nerve cell bodies and looks grey to the naked eye.They found that people with high IQ scores had significantly more grey matter in 24 of the regions than people with lower scores. Many of the areas, which are spread throughout the brain, are known to be related to memory, attention and language. Their results are reported online in Neuroimage1.

Haier believes that different aspects of intelligence might depend on the amount of grey matter in these different brain regions.

"This may be why one person is quite good at mathematics and not so good at spelling, and another person, with the same IQ, has the opposite pattern of abilities," he says. However, Haier and his colleagues also found that only about 6% of the total grey matter in the brain seems to be related to IQ. Nerve cells in these particular areas may work to allow the brain to process information more efficiently, suggests Haier.

"There is a constant cascade of information being processed in the entire brain, but intelligence seems related to an efficient use of relatively few structures, where the more grey matter the better," he says.

Cautious reception

The finding is intriguing, according to Robert Plomin, who studies intelligence at the Institute of Psychiatry, King's College London.

Out of all the possible brain characteristics that could be linked to intelligence, "it's surprising to find that the simplest of all these measures, brain-region size, is the most highly correlated," he says. That said, the correlation is not huge, he adds. Intelligence researcher John Duncan from the Cognition and Brain Sciences Unit at Cambridge adds a further note of caution, pointing out that it is still possible the correlation the researchers saw is due to chance. "It's difficult to say exactly what the study means," he says.